(a) Field of the Invention
The present invention relates to a thin film transistor liquid crystal display, more specifically to a thin film transistor liquid crystal display whose black matrix is formed on a thin film transistor substrate.
(b) Description of the Related Art
Most liquid crystal displays include a thin film transistor (TFT) substrate and a color filter substrate. Black matrix is generally formed on the color filter substrate and is used to shield the light leakage in the portions between pixels. However, misalignment between the TFT substrate and the color filter substrate may make it hard to shield the light leakage perfectly. For that reason, a method of forming the black matrix on TFTs, which is called black matrix on TFT (BM on TFT), was recently suggested.
FIG. 1 illustrates a cross-sectional view of a conventional BM on TFT type TFT substrate.
As shown in FIG. 1, a gate electrode 2 and a storage capacitor electrode 3 are formed on a transparent substrate 1. A gate insulating layer 4 is formed on the gate electrode 2 and the storage capacitor electrode 3. An amorphous silicon layer 5, an etch stopper layer 6 and an n+ amorphous silicon layer 7 are deposited sequentially on the gate insulating layer 4 over the gate electrode 2. A source electrode 8 and a drain electrode 9 are formed on the n+ amorphous silicon layer 7, and the source electrode 8 is connected to a data line (not shown). The gate electrode 2, the gate insulating layer 4, the amorphous silicon layer 5, the n+ amorphous silicon layer 7, the source electrode 8 and the drain electrode 9 form a TFT. A passivation layer 10 is formed on the TFT and the gate insulating layer 4, and a black matrix 11 is formed on the passivation layer 10 over the TFT. A pixel electrode 12 made of ITO (indium tin oxide) is formed on the passivation layer 10 in a pixel region, and connected to the drain electrode 9 through a contact hole in the passivation layer 10.
Because the pixel electrode 12 is close to the data line, coupling capacitance is generated between the pixel electrode 12 and the data line when the liquid crystal display is in operation, and the coupling capacitance distorts the display signal.
Since the black matrix 11 is formed on the TFT, the height difference between the portions near the TFT and the pixel electrode 12 can become larger to make defects of the alignment layer, thereby causing leakage. Although the light leakage may be reduced by increasing the width of the black matrix, in this case, the aperture ratio may decrease.
On the other hand, liquid crystal displays comprise two spaced parallel substrates and a liquid crystal layer therebetween. Spacers are inserted between the substrates to keep the cell gap, which is the thickness of the liquid crystal layer injected between two substrates, to be constant. It is common to use spherical spacers having uniform size, and the spacers are uniformly distributed on the pixel electrode 12. Because of the height difference in the color filter substrate and in the TFT substrate, it may be difficult to make a uniform cell gap. Therefore, the thickness of the liquid crystal layer becomes non-uniform, and display characteristics become worse. Moreover, the spacers on the pixel electrode 12 may cause a defect in the alignment layer and may cause the light from the backlight unit to be scattered, thereby causing the low transmittance of the liquid crystal cell and the light leakage.